Mild steel, nickel or its alloys are used as cathodes in hydrogen evolving systems. Cathode overvoltage losses are quite substantial in electrolysis of aqueous solutions. For example, hydrogen overvoltage in a non-mercury chlor-alkali cell employing mild steel cathode is of the order of 350-450 millivolts at a current density of 250 mA.cm.sup.-2 at 80.degree.-90.degree. C.
One form of such non-mercury type chlor-alkali cell which is currently in use is diaphragm cell in which anode and cathode compartments are separated by a diaphragm through which the electrolyte percolates from anode compartment to the cathode compartment and caustic soda is formed at the cathode. Another form of non-mercury type chlor-alkali cell is ion-exchange membrane cell where the asbestos diaphragm is replaced with ion-exchange membrane which allows only cations to pass through so as to produce high purity, higher concentration of caustic soda in the cathode compartment. In these type of chlor-alkali cells, generally steel cathodes are used, which have high overvoltage. Due to frequent increases in the cost of electrical energy, more attention has been paid to the development of a suitable catalytic cathode which will have minimum overvoltage and long term stability. Any reduction in this cathodic overvoltage will result in a substantial power saving. Many attempts have been made to reduce the overvoltage of hydrogen evolving cathodes. One such improvement is the development of electrodes made up of steel and like coated with various materials such as nickel, nickel-iron, nickel-zinc over them by electroplating. U.S. Pat. Nos. 4,033,837 and 4,105,531 disclose a method for the electroplating of Ni--Mo--V alloy over a conductive substrate such as steel. This material had somewhat lower overvoltage than uncoated steel, but suffered from corrosion and degradation problems.
Another attempt to produce a catalytic cathode for hydrogen evolution in alkaline solutions is described in U.S. Pat. No. 3,962,844. This process involves the deposition of amorphous borides of nickel, cobalt or iron. These cathodes can only be used at temperatures as low as about 20.degree. C. which is well below the general industrial operating temperatures which are commonly in the range of 80.degree.-90.degree. C. It appears that degradation of the material would take place at higher temperatures. So these cathodes have not been accepted for commercial production.
Another attempt to prepare a catalytic cathode is the development of "Raney Nickel". The process for forming a Raney Nickel catalyst over a metallic substrate such as steel or nickel is described in U.S. Pat. No. 4,116,804. The process involves plating and flame spraying of layers of nickel and aluminium respectively on the substrate followed by heating at a higher temperature to cause interdiffusion of the metals. The interdiffused aluminium is then leached out to give high surface area "Raney Nickel". Raney Nickel cathodes lack mechanical stability and are pyrophoric in nature and delamination of coating from the substrate occurs. It was found that Raney Nickel catalyst is oxidised to nickel hydroxide and becomes deactivated by the reverse current which flows during short circuit and cell shut downs. This increases the overvoltage of the cathode. Thus they have not been widely accepted for industrial use.
Japanese patent 80 12,687 describes a Raney nickel type cathode comprising of a precious metal. Such type of cathodes could not withstand current reversals, the occurence of which can not be avoided in industrial practice.
Japanese patent 80 50,478 teaches a method for the preparation of a cobalt composite coating containing nonionic and cationic polymers. As the coating consists of one component only, these electrodes were not industrially successful.
Japanese patents 80 131,189 and 81 41,395 disclose a one component system containing catalyst alone and a two component system containing catalyst and stabiliser respectively. Both these cathodes cannot maintain constant cathodic potential due to the absorption of hydrogen on the electrode surface. European patents EP 129,088 and EP 129,734 disclose cathodes which comprise a two component system. In the absence of a third component, which is hydrogen, overvoltage reducing metal/oxide especially gold or platinum, reduction in hydrogen overvoltage is not remarkable; they also lack long term stability.
A catalytic cathode material is disclosed in European patent (EP. 240,413) which comprises the deposition of one or more precious metal or precious metal oxide and one or more metal layer (eg. Ni or Ni--P alloy) over a conducting metallic substrate. In the course of operation as a cathode material in NaOH electrolysis, the nickel in the electrode slowly absorbs hydrogen, getting reduced to nickel hydride which reduces the activity of the coating. So this electrode also suffers from long term instability. Another attempt to prepare a catalytic cathode for hydrogen evolution reaction is the preparation of Platinum-Ruthenium alloy which is described in U.K. Patent No. 2,074,190. This process comprises contacting the electrically conductive matrix (Nickel, Copper and alloys of Nickel and/or Copper including alloying metals such as Iron, Cobalt and/or Chromium) with an acidic aqueous solution of a platinum salt and a ruthenium salt, such that some of the metal of the matrix exchanges with platinum and ruthenium in the solution thereby causing deposition of platinum and ruthenium onto the matrix. Contacting the matrix with the solution is effected by dipping or spraying. The displacement deposition is spontaneous and is due to the matrix having an electrode potential above that of platinum and ruthenium. No reducing agent is present in the solution used for deposition.
By this process it is very difficult to build deposits thicker than 0.5 microns. Even if the deposit thickness is built up by some other technique, the resultant cathode will be uneconomically costlier, as the deposit contains only precious metals. Though cathodes prepared as per the above said patent exhibit low hydrogen overvoltage they lack long term stability; the most probable reason for this characteristic may be the thin electrocatalytic deposit loosely bound to the matrix. U.S. Pat. No. 5,035,789 discloses yet another method for the preparation of catalytic cathodes having a coating consisting of a two component system made of a precious metal and non-precious transition metal. The coating in this case is obtained by electroless deposition. Though these electrodes exhibit low hydrogen overvoltage they do not have long term stability, as the electrocatalyst is loosely bound to the substrate. As an improvement over this, U.S. Pat. No. 5,066,380 suggests a thermal treatment for the electrodes wherein the electrocatalysts are deposited by non-electrolytic reduction. Here the high temperature treatment given to the electrodes induces thermal stress at the interface between the coating and the substrate as their thermal expansion coefficients are different and these electrodes also do not have a long active life.
The object of the present invention is to provide a cathode useful for the electrolysis of aqueous alkali metal halide solutions overcoming the disadvantages of the prior art cathodes used in systems evolving hydrogen. The invention also provides a process for the preparation of catalytic cathode.
We have observed that if the substrate is given a coating consisting of at least an oxide of a non precious transition metal for example Nickel oxide one or more metal/oxide selected from Ru, Rh, Ir, Pd and Os with metal/oxide of Gold or Platinum or both, then the resulting cathode has high catalytic activity and exhibits low hydrogen overvoltage.